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Home » Page 1781

Quantum well energizes nanocrystals

The non-radiative energy transfer process indirectly injects electron-hole pairs into the nanocrystals. These later recombine, emitting light as a result.

“The high efficiency of energy transfer in combination with the exceptional luminescent properties of nanocrystal quantum dots make hybrid quantum-well/nanocrystal devices feasible as efficient sources of any colour light – or even white light,” said Victor Klimov of Los Alamos.

In this study the scientists chose to impart energy to the quantum well by optical pumping so that they could obtain additional information about how that energy then transferred to the nanocrystals. In real-life, however, they say they could also pump the quantum well electrically, in the same way that a common quantum-well light-emitting diode is pumped.

The researchers used a 3 nanometre indium-gallium-nitride quantum well underneath a monolayer of cadmium-selenium/zinc sulphide core/shell nanocrystals. The nanocrystals, which had a core radius of 1.9 nanometres and a shell thickness of around 0.6 nanometres, were capped with organic molecules.

Klimov and colleagues pumped the quantum well by shining laser light with a wavelength of 266 nanometres on the device. The well transferred its energy to the nanocrystals with an efficiency of around 55%. And, according to the researchers, the nanocrystal capping layer did not significantly inhibit the process.

“The transfer of energy is fast enough to compete with exciton — bound electron-hole pair — recombination in the quantum well, and that allows us to ‘move’ more than 50% of the excitons to adjacent quantum dots,” said Klimov. “The recombination of these transferred excitons leads to the emission of light with colour that can be controlled by quantum-dot size.”

What’s more, the researchers believe they may be able to achieve efficiencies of nearly 100% by improving the quality of the quantum wells to reduce non-radiative losses, and by optimizing the geometry of the nanocrystal/quantum-well structure. The scientists say that the technique could also find a use in nanocrystal-based optical amplifiers and lasers.

Microscopy moves to 4D

“We are now able to see in-situ the full three dimensional shape of a single grain as it evolves during recrystallization in the interior of a sample,” says Soeren Schimdt of the Riso National Laboratory in Denmark. “One could say that we have a full four dimensional (4D) description of the grain. This is the first time ever this has been obtained.”

The four-dimensional microscope is a modified version of the Three Dimensional X-Ray Diffraction (3DXRD) microscope at the European Synchrotron Radiation Facility (ESRF) in Grenoble. Schimdt and colleagues from Riso and the ESRF used a beam of 50 keV X-rays from the ESRF to study a single crystal of aluminium that had been deformed enough to ensure that recrystallization would occur.

The Danish scientists started by positioning their sample in the 3DXRD microscope so that the X-ray beam intersected a single recrystallizing grain. Next, they made a series of vertical cuts across the grain and took images of the successive cross-sections. By then stacking these images together, they were able to obtain a full three-dimensional image of the grain. The procedure was repeated several times to produce a movie of the process (see Related links).

The team now plans to investigate recrystallization in other situations. “Hopefully we can combine these results and come up with a more fundamental description of the recrystallization process,” says Schmidt.

Old galaxies shed new light on young universe

According to the hierarchical model large galaxies are formed as a result of smaller galaxies merging. The model predicts that massive galaxies should appear rather late in the history of the universe, when it was about half its present age. Astronomers can calculate how old a galaxy is by measuring its redshift: galaxies with large redshifts are older and further away than those with small redshifts.

However, galaxies with high redshifts are difficult to detect because they are very faint and because the radiation they emit is shifted to longer wavelengths that are easily absorbed by the Earth’s atmosphere. This means that although astronomers can now routinely detect bright star-forming galaxies at redshifts up to 6.6, the most distant massive galaxy observed to date is still an object with a redshift of 1.552 that was discovered 10 years ago.

To overcome this problem, Andrea Cimatti of the Istituto Nazionale di Astrofisica (INAF) in Italy and co-workers searched for massive galaxies (1011 to 1012 solar masses) at redshifts greater than 1.5 by comparing their spectra with those of massive galaxies found in the present-day universe (Nature 430 184). Working with the Very Large Telescope at the European Southern Observatory in Chile, Cimatti and co-workers performed a survey of an area of the sky known as the Hubble Ultra Deep Field and identified four massive spheroid-shaped galaxies at redshifts between 1.6 and 1.9.

Meanwhile, Karl Glazebrook from John Hopkins University and colleagues, working with the Gemini telescope in Hawaii, studied a new sample of 150 galaxies in the so-called Gemini Deep Deep Survey (Nature 430 181). These galaxies have redshifts of between 0.8 and 2 and date back to just three billion years after the big bang. Glazebrook and co-workers found that a significant fraction of the galaxies with redshifts greater than 1.5 were massive.

US team breaks power density record

HERCULES is a custom-made titanium:sapphire laser that uses chirped pulse amplification (CPA) to generate ultrashort pulses with a power of 45 terawatts (27 femtosecond pulses containing 1.2 joules of energy). The Michigan team used adaptive optics, a well known technique for correcting wave-front distortion, to focus their pulses to the smallest spot possible. Pulses from HERCULES were reflected off a deformable mirror before being focused down by a paraboloid mirror.

The shape of the deformable mirror was adjusted to correct for any distortions in the beam and optimize the focusing. Using the technique the team managed to generate focused intensities of between 0.66 and 0.85×1022 watts per centimetre squared.

“It is the laser wave-front fluctuations that will ultimately limit the focused intensity and its spatial resolution,” the team told delegates at the post-deadline session of CLEO 2004 in San Francisco in May. “In our case, HERCULES is remarkably stable. The shot-to-shot fluctuation of wave-front has an rms (root mean square) deviation from its average shape of about 1/20.”

Gerard Mourou, the Center’s director, invented CPA in the late 1980s as a way to amplify pulses to much higher powers. The technique involves stretching an ultrashort pulse to several nanoseconds so that it can amplified without its peak power damaging the gain medium. After amplification the pulse is recompressed to give a very powerful ultrashort pulse.

New shift seen in cyclotrons

Cyclotrons are routinely employed to compare the masses of molecules and study chemical reaction rates. The cyclotron frequency is given by the simple formula qB/m, where q and m are the charge and mass of the particle and B is the magnetic field. However, Pritchard and co-workers have discovered that this formula needs to be tweaked for particles that can be polarized. This happens because an electric dipole moment is induced in the particle and the two ends of the dipole move at slightly different speeds. The end result is that the cyclotron frequency is shifted by a small amount.

The MIT team found that this frequency shift was about 1 part in 109 for a CO+ ion. Moreover, since the polarizability of the ion depends on its internal quantum state, the cyclotron frequency changes whenever the ion jumps to a different internal state. By measuring these changes, Pritchard and co-workers were able to calculate the dipole moment of the molecule. They were also able to determine the quantum state of the molecule by simply measuring its cyclotron frequency.

The team
The team

“This is a very general effect that had not been recognized before, and it needs to be taken into account in various experiments,” team member James Thompson told PhysicsWeb.

These include fundamental tests of CPT (charge-parity-time reversal) symmetry and experiments that use molecular ions to search for an electron dipole moment.

Critical reflections

When I wrote my first “Critical Point” in May 2000, I imagined that in each essay I would gently edify readers on various aspects of research outside physics that are relevant to events and issues in it. The result was not what I expected. I turned out to be the one who was edified.

“Edified” is not really the right word. More accurately, I got exposure. I got it the way that actors and advertisers mean it, which can never be bad. I got it in the sense that lawyers use the term, which can never be good. And I got it in the medical sense, like the bracing and invigorating – and only potentially dangerous – effects of a brisk walk in the cold.

This has taught me a few lessons about column writing.

Listen to your readers

While writers may think and act as if they are producers and the readers consumers, that is bad practice. The readers of a column are invariably more diverse, experienced and knowledgeable than its author. The first lesson I learned was to look for opportunities to tap into this resource.

For instance, after I had written only a few columns a reader asked me about the philosophical commitments of physicists: was it true, he asked, that almost all physicists are critical realists, as physicist-turned-Anglican priest John Polkinghorne had claimed in a recent book? Maybe, the reader suggested, I should use the column to poll physicists.

Being well-versed in the philosophy of science, I had my own opinions about the philosophical commitments of physicists. But perhaps, I thought, this reader was right and I ought to generate statistics.

I knew that if I simply listed the names of philosophical positions – idealism, instrumentalism and so on – readers’ responses would depend on their prior assumptions about the position, thus biasing the data.

So in my poll (October 2001 p18) I set up a webpage that listed a series of entities – the Earth, colours, atoms and so forth – and asked readers to tick which ones they thought were real and which not. I wrote up the results in a subsequent column (April 2002 pp15-17).

The success of this exercise led to other “interactive” columns on beautiful experiments (May 2002 p17) and physics humour (September 2003 p19; December 2003 pp14-15).

These columns generated exposure in the form of articles in the New York Times, the Sunday Telegraph and the Toronto Globe and Mail; led to me appearing several times on BBC radio; and spawned my book The Prism and the Pendulum: The Ten Most Beautiful Experiments in Science (2003 Random House).

Have a thick skin

I knew that political columnists receive harsh feedback from a tiny fraction of their readers, but thought this unlikely with the Physics World audience. I was wrong.

One reader was “horrified” when I used the word “pendulums” in an article about Léon Foucault’s device. This was so contemptible, the professor continued, that “it made the article impossible for me to read”. I pointed out that the Oxford English Dictionary (OED) lists “pendulums” as correct, and “pendula” as “formerly (rarely)” the plural form. Light-heartedly, I asked whether he might not be referring to a different kind of pendulum, noting that the force of gravity seemed to be somewhat stronger in his academic environs. I also suggested a compromise plural – “pendul*” – and asked whether he would read my article if I used it. But my correspondent would have none of it, and said I should know that the OED is an agent of cultural decay.

And he was one of the more polite complainers. I have been accused of “censorship”, “arrogance”, “stupidity” and “high self-importance”. Others accused me of holding back the progress of physics, costing it jobs and endangering its existence by not using the column to promote or denounce this or that idea or theory. The column on “Crackpots and their convictions” (May 2001 p14) generated especially hostile mail from individuals I had never heard of, some of whom thought I was targeting them personally and trying to destroy their careers. Two darkly hinted at impending legal action.

Have a hard heart

Every scientist, I suppose, fantasizes at some point about making a discovery that changes the world. Every writer has a similar fantasy, about writing something that sparks developments that matter.

I occasionally entertain such thoughts in connection with what I think of as my “outrage” columns. In them I vent anger at episodes in which important scientific facilities were destroyed, or leading scientists smeared, by activists who, disguised as progressives but motivated by reactionary politics, have succeeded in cowing government agencies while the rest of the scientific community stands by all but idle (May 2003 p19; January 2002 p17 and September 2001 p18).

Many of these columns ended with something like, “If you don’t care about what’s happening here, you don’t care about the future of science”. That thought threatens to become my version of “delenda est Carthago” (“Carthage must be destroyed” – the phrase with which the orator and politician Cato the Elder ended his speeches in the Roman Senate against the city’s rival). I secretly hoped that these columns would induce sufficient anger in enough scientists or administrators to foment some kind of cultural rebellion against pseudoscience and faux-progressivism, and spark changes in the planning and promoting of scientific projects.

Alas, the revolution is yet to happen.

The critical point

Each column generally culminates in a phase transition, in which I try to show how the subject or episode at hand transformed into a lesson about the relevance of research outside physics. For me, this remains the point. But saying this reminds me of a final lesson that I have learned by reading other columnists: never write about the column. In the end, a column is just another article in just another issue. Column writing is a far less profound and complex subject than physics.

I know I am violating this rule now, but I promise not to do it again. Unless, somehow, I manage to last another 50 columns. Meanwhile, I will hope for the revolution.

Entanglement breaks new record

By taking advantage of quantum phenomena such as entanglement, teleportation and superposition, a quantum computer could, in principle, outperform a classical computer in certain computational tasks. Entanglement allows particles to have a much closer relationship than is possible in classical physics. For example, two photons can be entangled such that if one is horizontally polarized, the other is always vertically polarized, and vice versa, no matter how far apart they are. In quantum teleportation, complete information about the quantum state of a particle is instantaneously transferred by the sender, who is usually called Alice, to a receiver called Bob. Quantum superposition, meanwhile, allows a particle to be in two or more quantum states at the same time.

Jian-Wei Pan at the University of Heidelberg in Germany and colleagues at the University of Science and Technology of China in Hefei and the University of Innsbruck in Austria began by producing a high intensity and ultra-stable source of entangled photons. Next they used two entangled pairs of photons to generate a four-photon entangled state, which they then combined with a single-photon state. They were able to produce a five-photon entangled state by detecting the coincidence of five photons.

To demonstrate open-destination teleportation, Pan and co-workers first teleported the unknown quantum state of a single photon onto a superposition of three photons. They were then able to read out this teleported state at any one of the three photons by performing a measurement on the other two photons.

“Although our experiment might seem to be only a modest step forward, the implications are profound,” Pan told PhysicsWeb. They plan to use their five-photon set-up to demonstrate “bit-flip error rejection” for quantum communication and to make a non-destructive controlled-NOT gate for quantum computation.

Could dark energy be studied in the lab?

Quantum fluctuations mean that the vacuum is not empty as is assumed in classical physics. These fluctuations, also known as zero-point fluctuations, are a consequence of the uncertainty principle, and they give the vacuum a structure that manifests itself in a variety of different ways such as the Casimir effect. Physicists have already measured the effects of this “vacuum energy” in circuits containing Josephson junctions.

A series of astrophysical observations have suggested that as much as 73% of the universe is made of dark energy — a gravitationally repulsive material that is causing the expansion of the universe to accelerate. However, no one knows what dark energy is made of. Vacuum energy is one candidate for dark energy, although the amount of energy in the vacuum predicted by theory is some 120 orders of magnitude more than the amount indicated by observations.

In 1982, Roger Koch and colleagues, then at the University of California at Berkeley and the Lawrence Berkeley Laboratory, performed an experiment in which they measured the frequency spectrum of current fluctuations in Josephson junctions. Their system was cooled to millikelvin temperatures so that thermal vibrations were reduced to a minimum, leaving only zero-point quantum fluctuations. Now, Christian Beck at Queen Mary University of London and Michael Mackey at McGill University in Montreal have re-analysed these results in the light of recent astrophysical estimates of the density of dark energy in the universe.

Beck and Mackey argue that the zero-point fluctuations measured by Koch’s team imply a non-zero density for the vacuum energy, and say that this value cannot exceed the value for the density of dark energy in the universe. Using this premise, they predict that there should be a cut-off in the spectrum of the fluctuations at a frequency of around 1.69 x 1012 Hertz.

Beck and Mackey believe that future experiments with a new generation of Josephson junctions that work at higher frequencies could help to clarify whether or not this cut-off exists. Such experiments would also show if dark energy is indeed related to vacuum energy.

Russian physicist scoops low temperature award

Volovik was chosen for the award for his pioneering research on the effects of symmetry in superfluids and superconductors and for extending these concepts to quantum field theory, cosmology, quantum gravity and particle physics.

“Grigory Volovik is an outstanding theorist who has shown how novel ideas and experiments from low-temperature physics might lead to a new understanding about the early universe and particle physics, in a unique synthesis,” said Mike Lea, Chair of the selection panel for the prize. Volovik, who recently wrote a book on his work called The Universe in a Helium Droplet, will receive the award at a Simon Memorial Prize conference in London in September. At this conference, he will also present a lecture called “Emergent physics: a condensed matter primer”.

In the past 30 years, five recipients of the Simon Memorial prize have gone on to win Nobel prizes, including last year’s winner Anthony Leggett, who won the Simon prize in 1981.

New look for molecular photodiodes

Molecular electronics has made steady progress in recent years and components made from single molecules could overcome the limits of conventional, silicon-based microelectronics. However, many challenges still need to be overcome before molecular electronic devices can become a reality, including the fact that it is difficult to integrate them into ordered structures.

The photodiode fabricated by Kimura and colleagues consists of two types of helical peptide — long protein chains — each of which has a different light-absorbing end-group, or chromophore. The two peptides, which are about 1 nanometre across, were anchored on a gold substrate. Such molecules are good candidates for molecular devices because they can form highly ordered self-assembled monolayers.

Kimura's team
Kimura’s team

The Kyoto team found that when one of the chromophores was excited with light of a certain wavelength it generated an anodic photocurrent. However, when the other chromphore was excited — with light of a different wavelength — the current flowed in the opposite direction, towards the cathode. The reason for this behaviour is that each helical peptide has a large intrinsic dipole moment that accelerates electron transfer in the same direction in which its dipole moment is pointing. Since the two peptides have dipole moments that point in opposite directions, the current is sent in opposite directions (see figure).

“The large dipole moment of helical peptides means they could be used as modulators in many types of electronic device made on the nanoscale,” Kimura told PhysicsWeb. “They could thus be useful starting materials for the coming age of molecular electronics.” The team now hopes to make a molecular transistor using such peptides.

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